Messages in this thread | | | Subject | Re: [question] sched: idle_avg and migration latency | From | Mike Galbraith <> | Date | Tue, 10 Dec 2013 16:11:42 +0100 |
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On Tue, 2013-12-10 at 12:30 +0100, Daniel Lezcano wrote: > Hi All, > > I am trying to understand how is computed the idle_avg and how it is > used regarding the migration latency. > > 1. What is the sysctl_sched_migration_cost value ? It is initialized to > 500000UL. Is it an arbitrarily chosen value ? Could it change depending > on the hardware performances ?
Yeah, it's a magic number. We used to use boot time measurements.
> 2. The idle_balance function checks: > > if (this_rq->avg_idle < sysctl_sched_migration_cost) > return 0; > > IIUC, it is not worth to migrate a task to this cpu as we expect to run > another task before we can pull a task to the current cpu, right ?
No, that's all about not beating living hell outta ourselves on every micro-idle. As with all load balancing, it's usually too much balancing that creates a problem. You need it, but it's really expensive, so less is more.
> Then if there is no task to balance we will enter idle, thus we > initialize the idle_stamp to the current clock. > > When another task is woken up with the ttwu_do_wakeup, the duration of > the idle time is computed in there: > > if (rq->idle_stamp) { > u64 delta = rq_clock(rq) - rq->idle_stamp; > u64 max = 2*sysctl_sched_migration_cost; > > if (delta > max) > rq->avg_idle = max; > else > update_avg(&rq->avg_idle, delta); > rq->idle_stamp = 0; > } > > Why is the 'delta' leveraged by 'max' ?
That has changed a little recently. I originally slammed avg_idle itself straight to max to ensure that a bursty load would idle balance, and not use stale data. If you start cross core switching at high frequency, you'll still shut idle balancing quickly.
> 3. And finally the function update_avg does: > > s64 diff = sample - *avg; > *avg += diff >> 3; > > Why is diff >> 3 used instead of the number of values ?
Ingo's quick like bunny smooth average.
-Mike
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